Archaeologists have revived the debate over whether a spectacular Bronze Age disc from Germany is one of the earliest known calendars.

The Nebra disc is emblazoned with symbols of the Sun, Moon and stars and said by some to be 3,600 years old.

Writing in the journal Antiquity, a team casts doubt on the idea the disc was used by ancient astronomers as a precision tool for observing the sky.

They instead argue that the disc was used for shamanistic rituals.

But other archaeologists who have studied the Himmelsscheibe von Nebra (Nebra sky disc) point to features which, they say, helped Bronze Age people to track four key dates during the year.

The Nebra disc is considered one of the most sensational - and controversial - discoveries in archaeology in the past 10 years.

The artefact was allegedly found by two treasure hunters near the town of Nebra, Germany, in 1999.

The plain explanation is that you have four dates on the disc

Ernst Pernicka, University of Tuebingen

Police in the Swiss city of Basel arrested the treasure hunters in a sting, and they were eventually convicted.

The pair said they found the disc on a 252m-high hilltop called Mittelberg in the German federal state of Saxony-Anhalt.

While many scholars support its status as an object from the Bronze Age, it is claimed to be a fake by others, notably the German researcher Peter Schauer from the University of Regensburg.

"German archaeologists don't say clearly that this is a fake. They hide, thinking that the thunderstorm will blow over," Dr Schauer told BBC News.

In the latest study of the artefact, Emilia Pasztor of the Matrica Museum in Hungary and Curt Roslund of Gothenburg University in Sweden, worked from the basis that the artefact dates to about 1,800 BC - the Bronze Age.

They examined the possibility that the 32cm-wide disc could have been used as a precise calendrical device.

Two golden arcs on the outside of the disc may show how far the sunrise and sunset move along the horizon between winter and summer solstices.

The arcs are 82.5 degrees long, which is the angle the Sun is seen to travel between the high mid-summer sunset and the low mid-winter sunset.

The precise angle varies from place to place. But Professor Wolfhard Schlosser, from the University of Bochum, in Germany, has pointed out that 82 degrees corresponds to the journey of the sun at the specific latitude in Nebra.

As such, it could have been used as a calendrical tool by Bronze Age Europeans.

Differing interpretations

"It's a difficult question to answer, but I do not think it was used as an instrument used for observing objects in the sky," Curt Roslund, an astronomer at Gothenburg told BBC News.

"I can't find any evidence for this," he added.

Roslund and Pasztor argue that few features on the disc tend towards exact representation and that it is more likely to have been of symbolic value - perhaps used in shamanic rituals.

But Ernst Pernicke, from the University of Tuebingen, Germany, maintained that the disc was likely to have been used as a calendrical tool.

If you urinate on a piece of bronze and then hide it in the ground for a few weeks you can produce the same patina as on the disc

Peter Schauer, University of Regensburg

"The plain explanation is that you have four dates on the disc," he told BBC News.

"You have the summer and winter solstice from the bends on the side, a date in March and in September from the Pleiades star constellation.

Supporters of this interpretation have proposed that the cluster of seven gold spots on the disc represent the constellation known as the Pleiades.

In Antiquity, Pasztor and Roslund suggest that if the goldsmith intended to produce an accurate chart of the sky, he would have not have ignored the conspicuous nearby constellation of Orion, and the square of Pegasus to the right.

But the disc could also have been used to harmonise the lunar and solar calendars.

Ralph Hansen from the University of Hamburg, found that a calculation rule in ancient Babylonian texts which said that a thirteenth month should be added to the lunar calendar when one sees the moon in exactly the arrangement that appears on the Nebra disc.

In addition, the number of stars on the disc is 32, along with the Moon, that makes 33 objects in total. Intriguingly, 33 Moon years are equivalent to 32 Sun years.

Seasonal indicator

This information could have told farmers when to plant and harvest their crops.

"The Moon is better for short-term time measurements - but this means that festivals change dates each year. For a society whose survival is dependent on agriculture, these cannot be changed because they are dependent on sunshine," said Ernst Pernicka.

"For everyday calendrical purposes, you would use Moon years. But for designing when to plough fields and when to harvest, you use Sun years."

Because bronze cannot be dated directly, claims of an ancient date for the disc rest on several pieces of evidence. They include:

The copper in the disc suggests it came from the eastern Alps, the main mine for copper during the Bronze Age.

The gold was mined in the Carpathian basin, a common source for gold during the same period.

The style of swords said to have been found with the disc, along with radiocarbon dates for a wooden grip on one of the swords, also suggest a Bronze Age origin.

Corrosion has formed a crystalline "malachite" patina on the disc, suggesting it is old, and is unlike artificially corroded copper.

"We have searched about a dozen different types of evidence for indications of a fake. In the absence of any positive results, the probability that the disc is authentic is multiplied each time," said Dr Pernicka.

But for Peter Schauer, the disc's authenticity remains in question.

"The patina on all the pieces is different," he said, "If you urinate on a piece of bronze and then hide it in the ground for a few weeks you can produce the same patina as on the disc."